#23 Added fixtures for new set of specs.
#23 Correctly mark collisions as resolved if there were no collisions
#23 Added new columnsPerGroup option

CC @skidding
#23 Finding positions is now aware of the concept of sections
#23 Snapping to grid is now aware of the concept of sections
#23 Making itemSpansMoreThanOneSection public method
#23 Revamping first scenario (now dragging 10 to the right; added ids)
#23 Add matcher that does not depend on item order, but on their ids.
#23 Add test for dragging item 10 to the right
#23 Second pullToLeft should only be done in non-sections mode.
#23 Add API for moving all sections to the right, starting with a specified one

Optionally, it can skip an item that is already within the range of
sections. The use-case for that is dropping an item onto a section, and
the local collisions not being resolved.
#23 _findLeftMostPositionForItem now stops at page boundary whenever column sections are enabled
#23 Refactor code in pullToLeft that places items starting with their leftmost limit.
#23 Collision resolving works differently in column sections mode

It tries to resolve the collisions locally and then pushes all the
content to the right by preserving section integrity.
#23 In column sections mode, collision detection only tries candidates within the same section
#23 API for getting the section number of an item
#23 Added API for retrieving an item given multiple attributes.
#23 Resizing an item pushes it to the next section if it cannot fit in the current section anymore.
#23 Deleting empty sections after resolving collisions in column section mode.
#23 Update fixtures with newer ones that also include item id.